Plantainoside D Reduces Depolarization-Evoked Glutamate Release from Rat Cerebral Cortical Synaptosomes.

Inhibiting the excessive release of glutamate in the brain is emerging as a promising therapeutic option and is efficient for treating neurodegenerative disorders. The aim of this study is to investigate the effect and mechanism of plantainoside D (PD), a phenylenthanoid glycoside isolated from Plantago asiatica L., on glutamate release in rat cerebral cortical nerve terminals (synaptosomes). We observed that PD inhibited the potassium channel blocker 4-aminopyridine (4-AP)-evoked release of glutamate and elevated concentration of cytosolic Ca 2+ . Using bafilomycin A1 to block glutamate uptake into synaptic vesicles and EDTA to chelate extracellular Ca 2+ , the inhibitory effect of PD on 4-AP-evoked glutamate release was prevented. In contrast, the action of PD on the 4-AP-evoked release of glutamate in the presence of dl-TBOA, a potent nontransportable inhibitor of glutamate transporters, was unaffected. PD does not alter the 4-AP-mediated depolarization of the synaptosomal membrane potential, suggesting that the inhibitory effect of PD on glutamate release is associated with voltage-dependent Ca 2+ channels (VDCCs) but not the modulation of plasma membrane potential. Pretreatment with the Ca 2+ channel blocker (N-type) ω-conotoxin GVIA abolished the inhibitory effect of PD on the evoked glutamate release, as did pretreatment with the protein kinase C inhibitor GF109203x. However, the PD-mediated inhibition of glutamate release was eliminated by applying the mitochondrial Na + /Ca 2+ exchanger inhibitor CGP37157 or dantrolene, which inhibits Ca 2+ release through ryanodine receptor channels. These data suggest that PD mediates the inhibition of evoked glutamate release from synaptosomes primarily by reducing the influx of Ca 2+ through N-type Ca 2+ channels, subsequently reducing the protein kinase C cascade.